PURPOSE: To reduce the amount of consumption of synthetic resin material and obtain the tube having sufficient flexibility by a method wherein a tubular sheet is bent and both side ends of the tubular sheet are bonded to the lower end of a bulkhead sheet at one place.

CONSTITUTION: A T-type sheet 1 is formed into a tube through extrusion molding process (a), in which the T-type sheet 1 consisting of the belt-type tubular sheet 2 and a belt-type bulkhead sheet 3 hung at the center of the lower surface of the tubular sheet 2 are formed through extrusion molding, and a bonding process (c), in which both side ends of the tubular sheet 2 of the T-type sheet 1 molded by extrusion molding through the extrusion molding process (a) is bonded to the lower end of the bulkhead sheet 3. In the bonding process (c), both side ends of tubular sheet 2 of the T-type sheet 1 are abutted against the lower end of the bulkhead sheet 3 by bending both side ends so that the tubular sheet 2 are wound and bent into the direction of wrapping the bulkhead sheet 3 while the abutted both side ends of the tubular sheet 2 are pushed against the lower end of the bulkhead sheet 3 and whereby said three ends are bonded at one place.